Frequent sleep-related respiratory disturbances (SRRD) have been recognized to occur in as many as 70% of elderly and 15% of middle-aged subjects. Although it is widely agreed that subjects with obvious daytime sleepiness that occurs in association with severe obstructive sleep apnea (Respiratory disturbance indices (RDI) >30) benefit from treatment of their sleep disorders; there is no consensus (and a paucity of data) regarding treatment benefits in subjects with a less profound disorder. Rational utilization of health care resources for diagnosis and treatment of sleep apnea requires assessment of whether adverse health effects occur as a consequence of mild or moderate levels of SRRD, and whether any such health effects may be reversible with treatment. In this study, we propose to determine the degree to which neuro- psychological performance and general health status and function may be impaired in subjects with mild and moderate degrees of SRRD, as compared to subjects with minimal apneic activity. The degree to which improvement may occur following specific treatment will also be assessed. We propose to assess neuropsychological performance, sleepiness and general functional status in 330 subjects, including subjects with little apneic activity (RDI<5), mild activity (RDI 5-14), and moderate activity (RDI 15-25). Of these subjects, 75% will be selected from a clinic-based sample, and 25% will be recruited from an ongoing population-based study. 150 subjects with mild and moderate activity will be randomized to receive "conservative" medical therapy (CMT) or CMT plus nasal CPAP therapy. We will utilize simple statistics, as well as multivariate techniques, to determine the relationship between SRRD (and associated hypoxemia, sleep fragmentation, and physiological sleepiness) to: a) intellectual abilities, attention and vigilance, psychomotor performance, learning and memory, and executive functions, and b) to general health status and function. Potential benefits of treatment specific for sleep apnea in subjects with mild and moderate SRRD will also be determined after two months of CPAP therapy. These data will provide new information about the health effects of mild and moderate sleep apnea that are needed for appropriate clinical decision making and utilization of health care resources.